A heart is the center of a person's circulatory system. It includes a complex electromechanical system performing two major pumping functions. The left portions of the heart, including the left atrium and the left ventricle, draw oxygenated blood from the lungs and pump it to the organs of the body to provide the organs with their metabolic needs for oxygen. The right portions of the heart, including the right atrium and the right ventricle, draw deoxygenated blood from the organs and pump it into the lungs where the blood gets oxygenated. These mechanical pumping functions are accomplished by contractions of the myocardium (heart muscles). In a normal heart, the sinus node, the heart's natural pacemaker, generates electrical signals, called action potentials, that propagate through an electrical conduction system to various regions of the heart to excite myocardial tissues in these regions. Coordinated delays in the propagations of the action potentials in a normal electrical conduction system cause the various regions of the heart to contract in synchrony to such that the pumping functions are performed efficiently. Thus, the normal pumping functions of the heart, indicated by hemodynamic performance, require a normal electrical system to generate the action potentials and deliver them to designated portions of the myocardium with proper timing, a normal myocardium capable of contracting with sufficient strength, and a normal electromechanical association such that all regions of the heart are excitable by the action potentials.
The function of the electrical system is indicated by electrocardiography (ECG) with at least two electrodes placed in or about the heart to sense the action potentials. When the heart functions irregularly or abnormally, one or more ECG signals indicate that contractions at various cardiac regions are chaotic and unsynchronized. Such conditions, which are related to irregular or other abnormal cardiac rhythms, are known as cardiac arrhythmias. Cardiac arrhythmias result in a reduced pumping efficiency of the heart, and hence, diminished blood circulation. Examples of such arrhythmias include bradyarrhythmias, that is, hearts that beat too slowly or irregularly, and tachyarrhythmias, that is, hearts that beat too quickly. A patient may also suffer from weakened contraction strength related to deterioration of the myocardium. This further reduces the pumping efficiency. For example, a heart failure patient suffers from an abnormal electrical conduction system with excessive conduction delays and deteriorated heart muscles that result in asynchronous and weak heart contractions, and hence, reduced pumping efficiency, or poor hemodynamic performance.
A cardiac rhythm management system includes a cardiac rhythm management device used to restore the heart's pumping function, or hemodynamic performance. Cardiac rhythm management devices include, among other things, pacemakers, also referred to as pacers. Pacemakers are often used to treat patients with bradyarrhythmias. Such pacemakers may coordinate atrial and ventricular contractions to improve the heart's pumping efficiency. Cardiac rhythm management devices also include defibrillators that deliver higher energy electrical stimuli to the heart. Such defibrillators may also include cardioverters, which synchronize the delivery of such stimuli to portions of sensed intrinsic heart activity signals. Defibrillators are often used to treat patients with tachyarrhythmias. In addition to pacemakers and defibrillators, cardiac rhythm management devices also include, among other things, devices that combine the functions of pacemakers and defibrillators, drug delivery devices, and any other devices for diagnosing or treating cardiac arrhythmias. Efficacy of a cardiac rhythm management device is measured by its ability to restore the heart's pumping efficiency, or the hemodynamic performance, which depends on the conditions of the heart's electrical system, the myocardium, and the electromechanical association. Therefore, in addition to the ECG indicative of activities of the heart's electrical system, there is a need to measure the heart's mechanical activities indicative of the hemodynamic performance, especially with the patient suffers from a deteriorated myocardium and/or poor electromechanical association.
For these and other reasons, there is a need for monitoring both electrical and mechanical activities of the heart for diagnostic and therapy evaluation purposes.